Turbochargers are well known and widely used with internal combustion engines for the purpose of increasing power output, decreasing fuel consumption and emissions, and compensating for air density loss during operation at high altitudes. Generally, turbochargers supply an increased charge air supply for the combustion process than can otherwise be induced through natural aspiration by utilizing exhaust gas energy to drive an air compressor. This increased air supply allows more fuel to be burned, thereby increasing power and output not otherwise obtainable from an engine having a given cylinder displacement under natural aspiration conditions. Additional benefits include the possibility of using lower-displacement, lighter engines with corresponding lower total vehicle weight to reduce fuel consumption, and use of available production engines to achieve improved performance characteristics and to accomplish a combination of increased power with reduced fuel consumption.
Some turbocharger applications include the incorporation of an intercooler for removing heat (both an ambient heat component and heat generated during charge air compression) from the charge air before it enters the engine, thereby providing an even more dense air charge to be delivered to the engine cylinders. Intercooled turbocharging applied to internal combustion engines such as diesel engines has been known to significantly increase the power output of a given engine size, in comparison with naturally aspirated engines of the same engine displacement.
Additional advantages of turbocharging include improvements in thermal efficiency through the use of some energy of the exhaust gas stream that would otherwise be lost to the environment, and the maintenance of sea level power ratings up to high altitudes.
At medium to high engine speeds, there is an abundance of energy in the engine exhaust gas stream and, over this operating speed range, the turbocharger is capable of supplying the engine cylinders with all the air needed for efficient combustion and maximum power and torque output for a given engine construction. In certain applications, however, an exhaust stream waste gate, i.e., vent, is needed to bleed off excess energy in the engine exhaust stream before it enters the turbocharger turbine to prevent the engine from being overcharged. Typically, the waste gate is set to open at a pressure below which undesirable predetonation or an unacceptably high internal engine cylinder pressure may be generated.
However, the turbocharged engine suffers an inherent deficiency in that at very low engine speeds, there is insufficient gas energy in the exhaust stream as may be found at higher engine speeds to produce significant levels of air charge pressure, and this energy deficiency prevents the turbocharger from providing a significant and readily available level of boost in the engine intake air system at low engine speeds. As a result, there is appreciable time lag between the time when the throttle is opened for the purpose of accelerating the engine from low speeds, such as idle speed, and when the turbocharger rotor is running fast enough to produce enough air charge pressure (boost pressure) to produce the desired acceleration while eliminating noticeable amounts of smoke when the engine is accelerated. Fuel control devices, such as rack limiters or aneroid controls, have been employed in the prior art to limit the amount of fuel delivered to the engine cylinder until the turbocharger is running fast enough to deliver sufficient air to produce generally smoke-free combustion. However, these fuel limiting devices are known to cause a noticeably slower response to throttle opening (turbo lag), with a corresponding sluggishness in engine and vehicle response.
Turbochargers are capable of very efficient operation at the mass flow and pressure conditions for which they are designed. However, when operated beyond design flow and pressure parameters, the component efficiencies of the turbine and compressor decrease. For example, if a turbocharger is designed and optimized to run at peak efficiency at the maximum speed and load range of a particular engine, then the efficiencies of the turbine and compressor components will likely be compromised at low engine speeds. In order to increase the amount of air delivered to the engine at or below torque peak, the turbocharger turbine is matched to the low engine speed range by decreasing the throat area of the turbine casing to increase the gas velocity entering the turbine wheel. This increases the rotational speed of the turbocharger rotor, but also imposes a higher back pressure in the engine exhaust system which is detrimental to engine performance especially at higher engine speeds and loads.
Specifically, an undesirable consequence of reducing the turbine casing throat area is that the smaller throat area causes the turbocharger rotor to "over-speed" when the engine is run at maximum load and speed and produces an air charge pressure which may exceed design limits due to an excess of exhaust gas energy at high engine speeds and loads. This consequence necessitates the use of a "waste gate" or exhaust gas bypass valve in the exhaust system that vents a portion of the exhaust gas stream outside of the turbine and limits the rotor speed to a fixed value. As a result, the maximum air charge pressure over the high engine speed range is limited to maintain engine cylinder pressures at controlled levels while effectively preventing operation above those constrained levels.
Typically, however, turbochargers are matched to existing engines to have good performance in the middle of the engine speed range, usually at the speed where maximum torque is required and generally not at maximum engine speed and load. Accordingly, this matching procedure also generally requires a waste gate in the exhaust system to prevent turbocharger over-speed at maximum engine speeds and loads in the manner described above. Regardless of how the turbocharger is matched, however, there is still a deficiency in exhaust gas flow at engine speeds below torque peak and at low engine speeds including idle speed.
The turbo lag period and deficiency in performance of the turbocharger at low engine speed and during acceleration can be mitigated and, in many instances, virtually eliminated by using an external power source to assist the turbocharger in responding to engine speed and load increases. One such method is to use an external electrical energy supply, such as energy stored in d.c. batteries to power an electric motor attached to the turbocharger rotating assembly that applies torque to the turbocharger rotor to maintain or increase rotor speed at low engine exhaust flow rates in order to supply sufficient charge air to reduce smoke and emissions during acceleration of the engine.
Turbocharging systems with integral assisting motors are more completely described in our pending U.S. patent applications Ser. Nos. 08/529,672 and 08/680,671, such disclosures being incorporated herein by reference.
Accordingly, there is still a need for an improved internal combustion engine system which improves engine performance, low-speed engine response and reduced emissions characteristics of a conventional internal combustion engine and an improved turbocharging system for controlling and optimizing turbocharged engine performance.